1. Field of the Invention
This invention relates in general to mobile communications antennas, and, more particularly, to a retractable mobile communications antenna suitable for cellular telephone and standard AM or FM radio reception.
2. Description of the Related Art
Cellular telephones require an antenna for both transmission and reception of high frequency radio signals, usually in the range of 825 to 895 MHz.
With the proliferation of cellular telephones in mobile vehicles, cellular, or car phone antennas have become both a common sight and one easily recognizable by the casual observer as a short, approximately eight inches, radiating whip antenna.
Unfortunately, the criminal element has also recognized the cellular antenna as a signal that an expensive cellular telephone is to be found in the vehicle. An exploding theft rate for vehicles sporting the tell-tale cellular antenna is therefore not unexpected.
Since cellular telephone users are generally only consumers of a service and know little about the operation of the cellular system, their primary desire is that the cellular telephone system in their vehicles function flawlessly, and that it do so as inconspicuously as possible.
Accordingly, dedicated, readily noticeable cellular antennas have fallen out of favor, not only because of increasing theft insurance premiums, but also because they can mar the appearance of an otherwise fine exterior design of a modern vehicle.
In many vehicles, the AM-FM radio is connected to an antenna that can be extended and retracted automatically, whenever the radio is activated. Normally, when the vehicle is parked and unattended, the antenna is retracted and unnoticed. Accordingly, the answer to some of these concerns appears to be the retractable antenna assembly, the use of which completely conceals the fact that the vehicle has a cellular telephone.
Several attempts have been made to create a telescopic antenna assembly for the mobile cellular frequency range, but all have generally failed for a number of reasons.
U.S. Pat. No. 4,725,846 is representative of the prior art in retractable cellular mobile antennas for vehicles. In this patent, the cellular mobile antenna is merely disguised as a conventional antenna, but fails to address the problems created by the retractable nature of the cellular portion of the antenna with regards to feed point efficiency and changing installation conditions. Chief among these problems is the difficulty in the matching impedance in a retractable antenna between the transmission line and the antenna due to the movement of the antenna feed point. That is, the point where the balanced coaxial cable connects to the base of the antenna radiating element to transfer the signal between the cellular telephone transceiver and the antenna system.
Likewise, there has been considerable difficulty in providing an antenna assembly that has a sufficiently broadband response over the entire 70 MHz alloted to the cellular mobile service (824 to 894 Mhz).
The present invention solves these problems by providing an antenna suitable for broadband mobile communication in the cellular range that is both retractable when not in use and which resembles an ordinary AM or FM vehicle antenna. In fact, embodiments of the present invention can be used for both cellular communications and standard AM - FM broadcast band reception.